Uplink (UL) power control apparatus and method in broadband wireless communication system

ABSTRACT

An uplink (UL) power control apparatus and method in a broadband wireless communication system are provided. The Mobile Station (MS) includes a power controller for calculating a power compensation value using a last transmit power in a previous closed loop power control when a power control mode is changed to an open loop power control, and determining a transmit power according to the open loop power control using the power compensation value; and a transmitter for adjusting and transmitting the transmit power of a UL signal under control of the power controller.

PRIORITY

This application claims priority under 35 U.S.C. §119 to an applicationfiled in the Korean Intellectual Property Office on May 12, 2006 andassigned Serial No. 2006-42958, the contents of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an uplink (UL) power controlapparatus and method in a broadband wireless communication system, andin particular, to an apparatus and method for stably switching a ULpower control mode in the broadband wireless communication system.

2. Description of the Related Art

Research to provide users with varying Quality of Service (QoS) at ahigh data rate is an objective of the fourth generation (4G)communication systems. Specifically, research into the high rate supportservice to guarantee mobility and QoS in the 4G communication systems,such as wireless Local Area Networks (LAN) and wireless MetropolitanArea Networks (MAN) guaranteeing relatively high data rates, has beenunder way.

The Institute of Electrical and Electronics Engineers (IEEE) 802.16communication system adopts an Orthogonal Frequency DivisionMultiplexing (OFDM) scheme and an Orthogonal Frequency Division MultipleAccess (OFDMA) scheme to support the broadband transmission network inphysical channels of the wireless MAN system. Applying the OFDM/OFDMAscheme to the wireless MAN system, the IEEE 802.16 communication systemenables the high data transmission by transmitting the physical channelsignal using a plurality of subcarriers.

In the OFDMA communication system, uplink (UL) signals may act asexcessive interference to other Mobile Stations (MSs) or neighbor cellsaccording to the transmit power, or cause a decrease in the Base Station(BS) reception power. Thus, an appropriate power control is requiredaccording to the required Carrier to Interference and Noise Ratio(CINR).

Typically, the power control mode can be broadly classified into aclosed loop power control and an open loop power control.

The closed loop power control compensates for the UL transmit power ofthe MS under the control of the BS. However, the closed loop powercontrol may degrade the accuracy of the power control in the packetcommunication system. The BS determines the power control range usingthe CINR value of the packets received in the UL. Even when the BSoccasionally receives the packets and performs the power control forevery packet reception, the accuracy of the power control may degradebecause there arises a difference between the time of the UL packettransmission from the MS and the time of the power control directionfrom the BS.

In the open loop power control, on the assumption that the path loss ofthe uplink is equal to the path loss of the downlink (DL), the MSadjusts the transmit power of the UL signal by estimating the DL pathloss by itself. That is, the MS adjusts the UL power using the requiredCINR received from the BS, interference and noise level information ofthe uplink, and the DL path loss. The BS can additionally direct thefine adjustment to the MS based on the CINR value of the receivedpackets.

As discussed above, the open loop power control can greatly improve theaccuracy of the power control thanks to the MS's own transmit poweradjustment and the BS's additional power adjustment direction, comparedto the closed loop power control.

Therefore, the MS in the OFDMA communication system performs the closedloop power control in the initial network entry phase, and then performsthe open loop power control by switching the power control mode.

FIG. 1 depicts a conventional switching procedure from the closed looppower control to the open loop power control in a broadband wirelesscommunication system.

At the initial access, MS 10 of FIG. 1 receives Downlink ChannelDescription/Uplink Channel Description (DCD/UCD) messages from BS 20 instep 101, and acquires information (parameters) required for the initialaccess from the received messages. In doing so, the MS 10 can acquireparameters relating to the initial ranging. The MS 10 receives ULinterference and noise level information from the BS 20 in step 103.

In step 105, the MS 10 sets an initial transmit power based on theinformation received from the BS 20. In step 107, the MS 10 sends aninitial ranging code to the BS 20 with the initial transmit power. TheBS 20 sends a ranging response RNG_RSP message to the MS 10 in reply tothe initial ranging code in step 109. The BS 20 sends a band allocationmessage CDMA Alloc IE for the ranging request to the MS 10 in step 111.

When the RNG_RSP message is not received within a present time after theinitial ranging code is transmitted, the MS 10 retransmits the initialranging code at an increased transmit power level. When the RNG_RSPmessage is received within the preset time, the MS 10 performs NetworkEntry (N/E) procedures starting from the initial ranging in step 113.The N/E procedures include the initial ranging RNG_REQ/RSP, the basiccapability negotiation SBC_REQ/RSP, and the authentication PKM_REQ/RSP.

After the N/E procedures, the MS 10 enters the closed loop power controlmode which adjusts the transmit power according to power control IE fromthe BS 20 in step 115.

The BS 20 checks the open loop power control capability of the MS 10through the basic capability negotiation procedure. After the N/Eprocedures, the BS 20 requests the MS 10 to switch to the open looppower control mode by sending Power control Mode Change ReSPonse(PMC_RSP) message in step 117. The MS 10 transmits Power control ModeChange REQuest (PMC_REQ) message to the BS 20 in response in step 119,and changes the power control mode to the open loop power control instep 121.

The MS 10 calculates the transmit power P according to the open looppower control based on Equation (1).

$\begin{matrix}{P = {L + {C/N} + {NI} - {10\;{\log_{10}(R)}} + {Offset\_ SS}_{perSS} + {Offset\_ BS}_{perSS}}} & (1)\end{matrix}$

Parameters in Equation (1) are defined as below:

-   -   P: transmit power (dBm) per subcarrier of UL burst.    -   L: average estimation value for propagation path loss which is        calculated using the total receive power measured through active        subcarriers of the preamble and Efficient Isotropic Radiation        Power parameter from the BS (BS_EIRP). BS_EIRP parameter        indicative of the BS transmit power is received using a DCD        message.    -   C/N: received CINR value required by Modulation and Coding        Scheme (MCS) level of the UL burst.    -   NI: estimation value of the average interference and noise power        (dBm) per subcarrier measured at the BS, which is provided to        every MS as common information.    -   R: number of repetitions according to the MCS level    -   Offset_SS_(perSS): MS power compensation value controlled by the        MS, which is always zero in the passive open loop power control        mode.    -   Offset_BS_(perSS): BS power compensation value controlled by the        BS. When this value is set using PMC_RSP message,        Offset_BS_(perSS) is substituted with the value of PMC_RSP        message. When the BS directs the power fine adjustment using the        power control IE, the accumulated power adjustment values in the        power control IE are used as Offset_BS_(perSS) value.        Alternatively, the accumulation of the power adjustment values        in RNG_RSP message received from the BS can be used as        Offset_BS_(perSS) value.

The conventional art has a number of drawbacks.

When a Relay Station (RS) (or Repeater) is installed to the system, andparticularly when the transmit power of the RS downlink (RS→MS) differsfrom the transmit power of the RS uplink (RS→BS), the open loop powercontrol may not normally work. This is because the open loop powercontrol fundamentally assumes that the DL propagation path loss is equalto the UL propagation path loss.

If the RS DL transmit power is greater than the UL transmit power, theMS estimates the UL propagation path loss by measuring the DLpropagation path loss. Accordingly, the estimated UL propagation pathloss is less than the actual path loss. In this case, when the open looppower control is conducted based on Equation (1), the MS transmits theUL packet with much less power than the required transmit power. As aresult, the BS may not receive the UL signal or the error rate of the ULpacket may increase. Particularly, when the power control mode ischanged (closed loop power control→open loop power control) with thedifferent propagation path losses between the downlink and the uplink,its influence is considerable.

If the set BS_EIRP value of DCD is greater than the actual BS outputvalue, then the MS calculates the DL path loss greater than the actualvalue. In this case, since the unnecessarily large UL transmit outputmay be set in the open loop power control, this can exert influence onother MS signals in view of the BS reception. In other words, when theBS performs an Automatic Gain Control (AGC) before a Fast FourierTransform (FFT) stage, the AGC operates based on the summation of all ULsignal powers. Thus, this may act as the interference to the MS signalhaving the relatively weak receive signal.

By contrast, when the set BS_EIRP value of DCD is less than the actualBS output value, the calculated MS DL path loss becomes less than theactual value. In this case, since the less UL transmit output than isnecessary can be set in the open loop power control, the BS may notreceive the MS signal.

As discussed above, since the open loop power control can compute theimproper transmit power because of the various external factors, what isneeded is a method for stably accomplishing the open loop power control.Furthermore, such errors are notable when the closed loop power controlis changed to the open loop power control. Therefore, it is required toproperly maintain the transmit power when the power control mode ischanged.

SUMMARY OF THE INVENTION

An aspect of the present invention is to substantially solve at leastthe above problems and/or disadvantages and to provide at least theadvantages below. Accordingly, an aspect of the present invention is toprovide an apparatus and method for stably changing a power control modein a broadband wireless communication system.

Another aspect of the present invention is to provide an apparatus andmethod for stably changing from a closed loop power control mode to anopen loop power control mode in a broadband wireless communicationsystem.

A further aspect of the present invention is to provide an apparatus andmethod for stably performing an open loop power control in a broadbandwireless communication system.

The above aspects are achieved by providing a Mobile Station (MS) in awireless communication system, which includes a power controller forcalculating a power compensation value using a last transmit power in aprevious closed loop power control when a power control mode is changedto an open loop power control, and determining a transmit poweraccording to the open loop power control using the power compensationvalue; and a transmitter for adjusting and transmitting the transmitpower of an uplink (UL) signal under control of the power controller.

According to one aspect of the present invention, a UL power controlmethod in a wireless communication system includes calculating a powercompensation value using a last transmit power in a previous closed looppower control when a power control mode is changed to an open loop powercontrol; and determining a transmit power according to the open looppower control using the power compensation value.

According to another aspect of the present invention, a UL power controlmethod in a wireless communication system includes acquiring, when apower control mode is changed to an open loop power control, a lasttransmit power value P_(Tx,CL) _(—) _(last) in a closed loop powercontrol mode, a path loss value L_(OL) _(—) _(init) estimated at aMobile Station (MS), a latest Noise and Interference (NI) value NI_(OL)_(—) _(init) received from a Base Station (BS), a required CINR valueC/N_(CL) _(—) _(last) for a last Modulation and Coding Scheme (MCS)level in the closed loop power control mode, and a number of repetitionsR_(CL) _(—) _(last) according to the last MCS level; and calculating apower compensation value Offset_SS_(perSS) controlled by the MS usingthe acquired values based on

Offset_SS_(perSS) = P_(Tx, CL_last) − (L_(OL_init) + NI_(OL_init)) − C/N_(CL_last) + 10 log₁₀(R_(CL_last)).

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 illustrates a conventional procedure for switching from a closedloop power control to an open loop power control in a broadband wirelesscommunication system;

FIG. 2 illustrates Mobile Station (MS) in a broadband wirelesscommunication system according to the present invention; and

FIG. 3 illustrates a procedure for performing an uplink (UL) open looppower control in the broadband wireless communication system accordingto the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described hereinbelow with reference to the accompanying drawings. In the followingdescription, well-known functions or constructions are not described indetail since they would obscure the invention in unnecessary detail.

The present invention provides a method for stably performing an openloop power control in a broadband wireless communication system.

As discussed earlier, in the open loop power control based on Equation(1), the improper transmit power is calculated when the path losses ofthe uplink and the downlink are not equal to each other. Typically, whenRelay Station (RS)(or Repeater) is used between Base Station (BS) andMobile Station (MS), the simplest method for normally accomplishing theopen loop power control is to set the DL gain and the UL gain of the RSto the same value.

The drawbacks of the open loop power control can be addressed bygradually increasing the transmit power in the bandwidth request rangingor the periodic ranging which is carried out before the actualtransmission of a UL burst, similar to the initial ranging.

The MS requests a UL bandwidth through the bandwidth request ranging. Inthe bandwidth request ranging, the transmit power is gradually increasedsimilar to the procedure used during the initial ranging. Specifically,the MS increases the transmit power of the bandwidth request ranginguntil a BS reception level reaches a proper level. Accordingly, in alater UL burst transmission, the BS reception level can be maintained atthe proper level.

The MS also performs the periodic ranging even while no UL packet istransmitted. In this case, like in the initial ranging, the transmitpower is gradually raised. Specifically, the MS increases the transmitpower of the periodic ranging until the BS reception level reaches aproper level. Accordingly, in a later UL burst transmission, the BSreception level can be maintained at a proper level.

The above methods address the drawbacks of the open loop power controlusing well-known techniques. Alternatively, a PMC_RSP message can beutilized. In more detail, the Offset_BS_(perSS) value of the PCM_RSPmessage is set to the difference between the DL gain and the UL gain ofthe RS. Since every MS connected to the BS and the RS enters the openloop power control mode and transmits signals with high power up to theOffset_BS_(perSS) value of the PMC_RSP message, interference on othercells may rise momentarily. However, the MS establishing a link to theBS can lower the transmit power using the constant power control IE, andthe MS establishing a link to the RS can maintain a certain receptionpower level in view of the BS reception.

Besides the above-mentioned methods, the MS can calculate by itself thereliable transmit power in the open loop power control, which isdescribed in detail by referring to the drawings.

FIG. 2 is a block diagram of MS in the broadband wireless communicationsystem according to the present invention. The following explanationdescribes a Time Division Multiplexing (TDD)-OFDMA system by way ofexample. Note that the present invention is easily applicable to everypower control system such as a Frequency Division Duplexing (FDD)-OFDMAsystem and a hybrid system using both TDD and FDD.

The MS of FIG. 2 includes a Media Access Control (MAC) layer part 201connected to an upper layer, a transmit modem 203, a receive modem 205,a duplexer 207, a power controller 209, and a receive power measurer211.

The MAC layer part 201 serves to receive transmit data from the upperlayer (e.g., IP layer part) and to provide the transmit data to thetransmit modem 203 by processing the transmit data according to aconnection scheme of the transmit modem 203. The MAC layer part 201receives receive data from the receive modem 205, processes and providesthe receive data to the upper layer according to a connection scheme ofthe upper layer. According to the present invention, the MAC layer part201 provides information required for the power control to the powercontroller 209. The information required for the power control caninclude information received from the BS, and information generatedbased on the information received from the BS.

The transmit modem 203 includes a channel coding block, a modulationblock, a Radio Frequency (RF) transmit block, and so forth. The transmitmodem 203 transforms data (burst data) fed from the MAC layer part 201to a form for the radio section transmission and provides thetransformed data to the duplexer 207. The channel encoding block caninclude a channel encoder, an interleaver, and a modulator. Themodulation block can include an Inverse FFT (IFFT) operator for mappingthe transmit data to a plurality of orthogonal subcarriers. The RFtransmit block can include a filter and an RF front-end unit.

The receive modem 205 includes an RF receive block, a demodulationblock, and a channel decoding block. The receive modem 205 restores datafrom the radio section signals from the duplexer 207 and provides therestored data to the MAC layer part 201. The RF receive block caninclude a filter and an RF front-end unit. The demodulation block caninclude an FFT operation for extracting data mapped to the subcarriers.The channel decoding block can include a demodulator, a deinterleaver,and a channel decoder.

The duplexer 207 provides the receive signal (DL signal) from an antennato the receive modem 205 and provides the transmit signal (UL signal)from the transmit modem 203 to the antenna according to the TDD scheme.

The receive power measurer 211 receives subcarrier values of thepreamble received from the BS from the receive modem 205, measures thereceive power using the preamble subcarrier values, and provides themeasured receive power to the power controller 209. The measured receivepower is used to calculate the UL path loss L of Equation (1).

The power controller 209 carries out the closed loop power control orthe open loop power control. In the closed loop power control, the powercontroller 209 determines the UL transmit power according to a powercontrol command received from the BS and provides the UL transmit powerto the transmit modem 203. The transmit modem 203 sends the UL signal byadjusting the transmit power of the UL signal according to the transmitpower from the power controller 209. The transmit power can be regulatedat one of a baseband stage, an Intermediate Frequency (IF) stage, and anRF stage.

In the open loop power control, the power controller 209 determines theUL transmit power based on Equation (1) and provides the determined ULtransmit power to the transmit modem 203. When the transmit power isdetermined based on Equation (1), information relating to the CINR valuerequired by the MCS level of the transmitted UL burst and the averageinterference and noise power estimation value (NI) per subcarrier at theBS are required, which are supplied from the MAC layer part 201.

When changing from the closed loop power control mode to the open looppower control mode, the power controller 209 computes the MS powercompensation value Offset_SS_(perSS) of Equation (1) based on Equation(2) to prevent the abrupt change of the transmit power according to thepower control mode switching, and performs the open loop power controlby reflecting the computed MS power compensation value into Equation(1).

$\begin{matrix}{{Offset\_ SS}_{perSS} = {P_{{Tx},{{CL}\_{last}}} - P_{{Tx},{{OL}\_{init}}} + {\Delta\;{CINR}_{req}}}} & (2)\end{matrix}$

The parameters in Equation (2) are defined as below.

-   -   P_(Tx,CL) _(—) _(last): last transmit power value in the closed        loop power control mode    -   P_(Tx,OL) _(—) _(init): initial transmit power estimation value        after changing to the open loop power control mode, which can be        expressed as Equation (3).

$\begin{matrix}{P_{{Tx},{{OL}\_{init}}} = {L_{{OL}\_{init}} + {C/N_{{OL}\_{init}}} + {NI}_{{OL}\_{init}} - {10\;{{\log_{10}\left( R_{{OL}\_{init}} \right)}.}}}} & (3)\end{matrix}$

-   -   ΔCINR_(req): a difference between the required CINR value for        the MCS level of the transmitted UL burst and the required CINR        value for the last MCS used in the closed loop power control        mode, which can be expressed as Equation (4).

$\begin{matrix}{{\Delta\;{CINR}_{req}} = {{C/N_{{OL}\_{init}}} - {C/N_{{CL}\_{last}}} - \left( {{10\;{\log_{10}\left( R_{{OL}\_{init}} \right)}} - {10\;{\log_{10}\left( R_{{CL}\_{last}} \right)}}} \right)}} & (4)\end{matrix}$

Accordingly, Equation (2) can be re-expressed as Equation (5).

$\begin{matrix}{{Offset\_ SS}_{perSS} = {P_{{Tx},{{CL}\_{last}}} - \left( {L_{{OL}\_{init}} + {NI}_{{OL}\_{init}}} \right) - {C/N_{{CL}\_{last}}} + {10\;{\log_{10}\left( R_{{CL}\_{last}} \right)}}}} & (5)\end{matrix}$

The parameters in Equation (5) are defined as follows. P_(Tx,CL) _(—)_(last) is the last transmit power value in the closed loop powercontrol mode. L_(OL) _(—) _(init) is the path loss estimated by the MSwhen switching from the closed loop power control to the open loop powercontrol. NI_(OL) _(—) _(init) is the latest Noise and Interference (NI)value received at the BS. C/N_(CL) _(—) _(last) is the required CINRvalue for the last MCS level (modulation and FEC) in the closed looppower control mode. R_(CL) _(—) _(last) is the number of repetitions(repetition factor) according to the last MCS level.

As above, the power controller 209 calculates the MS power compensationvalue Offset_SS_(perSS) based on Equation (2) or Equation (5), andperforms the open loop power control by reflecting the MS powercompensation value. As one can see from Equation (2), the MS powercompensation value of Equation (1) in the open loop power control is setto the sum of the difference between the last transmit power value inthe previous closed loop power control mode and the initial transmitpower value estimated by changing to the open loop power control mode,and the difference between the required CINR values based on the MCSdifference, rather than to zero.

The MS power compensation value in Equation (2) and Equation (5) iscalculated only once when the power control mode is changed from theclosed loop power control to the open loop power control, and then ismaintained until the power control mode is changed. Under the perfectpower control, since the initial power level of the open loop powercontrol is the same as the last power level of the closed loop powercontrol, the MS power compensation value Offset_SS_(perSS) will be zero.However, in the case of a disparity between the UL path loss and the ULpath loss, the abrupt change of the transmit power in the mode change isavoided by compensating for the disparity using the MS powercompensation value.

FIG. 3 illustrates a procedure for performing the UL open loop powercontrol in the broadband wireless communication system according to thepresent invention.

In FIG. 3, in step 301 the MS determines if the power control mode ischanged to the open loop power control. The power control mode ischanged using the PMC_RSP message received from the BS as describedearlier. Upon receiving the PMC_RSP message from the BS, the MSrecognizes the mode change request to the open loop power control,transmits the PMC_REQ message to the BS in reply to the PMC_RSP message,and then enters the open loop power control.

When entering the open loop power control mode, in step 303 the MSdetermines the last transmit power value in the closed loop powercontrol mode and the required CINR value for the last MCS level. In step305, the MS determines the required CINR value for the initial MCS levelof the UL burst which will be transmitted after changing to the openloop power control mode.

In step 307, the MS estimates the initial transmit power to be usedafter changing to the open loop power control mode based on Equation(3). In step 309, the MS calculates the MS power compensation valueOffset_SS_(perSS) using the last transmit power value P_(Tx,CL) _(—)_(last) of the closed loop power control mode, the required CINR valuefor the last MCS level, the required CINR value for the initial MCSlevel, and the initial transmit power estimation value P_(Tx,OL) _(—)_(init). The MS power compensation value can be acquired from Equation(2) or Equation (5).

After computing the MS power compensation value Offset_SS_(perSS), theMS performs the open loop power control by reflecting the MS powercompensation value into Equation (1) in step 311.

As set forth above, in a broadband wireless access communication systemsuch as an OFDMA system, when changing the power control mode to theopen loop power control mode, the open loop power control can be stablycarried out even with a disparity between the UL path loss and the DLpath loss. In other words, by keeping the proper receive power level ofthe BS for the UL packet transmission, the stable open loop powercontrol can be achieved and the UL performance can be enhanced.

While the invention has been shown and described with reference tocertain preferred embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims.

1. A Mobile Station (MS) in a wireless communication system, comprising:a power controller for acquiring a last transmit power value P_(Tx,CL)_(—) _(last) in a closed loop power control mode, a path loss valueL_(OL) _(—) _(init) estimated at the MS, a latest Noise and Interference(NI) value NI_(OL) _(—) _(init) received from a Base Station (BS), arequired Carrier to Interference and Noise Ratio value (CINR value)C/N_(CL) _(—) _(last) for a last Modulation and Coding Scheme (MCS)level in the closed loop power control mode, and a repetition factorR_(CL) _(—) _(last) according to the last MCS level, when a powercontrol mode is changed from a closed loop power control to an open looppower control, calculating a power compensation value Offset_SS_(perSS)using the acquired values, and determining a transmit power level Paccording to the open loop power control using the power compensationvalue; and a transmitter for adjusting a transmit power of an Uplink(UL) signal according to the determined transmit power level P.
 2. TheMS of claim 1, wherein the power controller calculates the powercompensation value Offset_SS_(perSS) based onOffset_SS_(perSS) = P_(Tx, CL_last) − (L_(OL_init) + NI_(OL_init)) − C/N_(CL_last) + 10 log₁₀(R_(CL_last)).3. The MS of claim 1, wherein the power controller determines a transmitpower level P according to the open loop power control based onP = L + C/N + NI − 10 log₁₀(R) + Offset_SS_(perSS) + Offset_BS_(perSS)where P is a transmit power (dBm) value per subcarrier of UL burst, L isan estimation value for path loss, C/N is a CINR value required by anMCS level of the UL burst, NI is a Noise and Interference (NI) power(dBm) estimation value at the BS, R is a number of repetitions(repetition factor) according to the MCS level, Offset_SS_(perSS) is anMS power compensation value controlled by the MS, and Offset_(—BS)_(perSS) is a BS power compensation value controlled by the BS.
 4. TheMS of claim 1, further comprising: a power measurer for measuring areceive power of a preamble received from the BS and providing themeasured receive power to the power controller, wherein the powercontroller estimates UL path loss using the BS transmit power valuereceived from the BS and the receive power fed from the power measurer,and uses the estimated path loss during the open loop power control. 5.The MS of claim 1, wherein the power control mode is changed using aPower control Mode Change ReSPonse (PMC_RSP) message from the BS.
 6. TheMS of claim 1, wherein the power compensation value Offset_SS_(perSS) iscalculated when the power control mode is changed, and is maintaineduntil the power control mode is changed.
 7. An uplink (UL) power controlmethod in a wireless communication system, the method comprising:calculating a power compensation value using a last transmit power in aprevious closed loop power control when a power control mode is changedfrom a closed loop power control to an open loop power control; anddetermining a transmit power according to the open loop power controlusing the power compensation value, wherein the power compensation valueOffset_SS_(perSS) is calculated based onOffset_SS_(perSS) = P_(Tx, CL_last) − P_(Tx, OL_init) + Δ CINR_(req)where P_(Tx,CL) _(—) _(last) is a last transmit power value in a closedloop power control mode, P_(Tx,OL) _(—) _(init) is an initial transmitpower estimation value after the change to the open loop power controlmode, and ΔCINR_(req) is a difference between a required Carrier toInterference and Noise Ratio (CINR) value for an initial Modulation andCoding Scheme (MCS) level in the open loop power control mode and arequired CINR value for the a last MCS level in the closed loop powercontrol mode.
 8. An uplink (UL) power control method in a wirelesscommunication system, comprising: acquiring, when a power control modeis changed from a closed loop power control to an open loop powercontrol, a last transmit power value P_(Tx,CL) _(—) _(last) in a closedloop power control mode, a path loss value L_(OL) _(—) _(init) estimatedat a Mobile Station (MS), a latest Noise and Interference (NI) valueNI_(OL) _(—) _(init) received from a Base Station (BS), a required CINRvalue C/N_(CL) _(—) _(last) for a last Modulation and Coding Scheme(MCS) level in the closed loop power control mode, and a number ofrepetitions R_(CL) _(—) _(last) according to the last MCS level; andcalculating a power compensation value Offset_SS_(perSS) controlled bythe MS using the acquired values; and determining a transmit power levelP according to the open loop power control using the power compensationvalue Offset_SS_(perSS).
 9. The UL power control method of claim 8,further comprising: adjusting a UL signal power level P to thedetermined transmit power and transmitting the UL signal.
 10. The ULpower control method of claim 8, wherein the power compensation valueOffset_(—SS) _(perSS) is calculated based onOffset_SS_(perSS) = P_(Tx, CL_last) − (L_(OL_init) + NI_(OL_init)) − C/N_(CL_last) + 10 log₁₀(R_(CL_last)).11. The UL power control method of claim 8, wherein a transmit powerlevel P according to the open loop power control is determined based onP = L + C/N + NI − 10 log₁₀(R) + Offset_SS_(perSS) + Offset_BS_(perSS)where P is a transmit power (dBm) value per subcarrier of UL burst, L isan estimation value for path loss, C/N is a CINR value required by anMCS level of the UL burst, NI is an NI power (dBm) estimation value atthe BS, R is a number of repetitions (repetition factor) according tothe MCS level, Offset_SS_(perSS) is an MS power compensation valuecontrolled by the MS, and Offset_BS_(perSS) is a BS power compensationvalue controlled by the BS.
 12. The UL power control method of claim 8,wherein the power control mode is changed using a Power control ModeChange ReSPonse (PMC_RSP) message from the BS.
 13. The UL power controlmethod of claim 8, wherein the power compensation valueOffset_SS_(perSS) is calculated when the power control mode is changed,and is maintained until the power control mode is changed.
 14. A powercontrol apparatus in a wireless communication system, comprising: meansfor acquiring, when a power control mode is changed from a closed looppower control to an open loop power control, a final transmit powervalue P_(Tx,CL) _(—) _(last) in a closed loop power control mode, a pathloss value L_(OL) _(—) _(init) estimated at a Mobile Station (MS), alatest Noise and Interference (NI) value NI_(OL) _(—) _(init) receivedfrom a Base Station (BS), a required CINR value C/N_(CL) _(—) _(last)for a last Modulation and Coding Scheme (MCS) level in the closed looppower control mode, and a number of repetitions R_(CL) _(—) _(last)according to the last MCS level; and means for calculating a powercompensation value Offset_SS_(perSS) controlled by the MS using theacquired values based onOffset_SS_(perSS) = P_(Tx, CL_last) − (L_(OL_init) + NI_(OL_init)) − C/N_(CL_last) + 10 log₁₀(R_(CL_last)).15. A Mobile Station (MS) in a wireless communication system,comprising: a power controller for calculating a power compensationvalue using a final transmit power in a previous closed loop powercontrol when a power control mode is changed from a closed loop powercontrol to an open loop power control, and determining a transmit poweraccording to the open loop power control using the power compensationvalue; and a transmitter for adjusting and transmitting the transmitpower of an uplink (UL) signal under control of the power controller,wherein the power controller calculates the power compensation valueOffset_SS_(perSS) based onOffset_SS_(perSS) = P_(Tx, CL_last) − P_(Tx, OL_init) + Δ CINR_(req)where P_(Tx,CL) _(—) _(last) is a final transmit power value in theclosed loop power control mode, P_(Tx,OL) _(—) _(init) is an initialtransmit power estimation value after the change to the open loop powercontrol mode, and ΔCINR_(req) is a difference between a required Carrierto Interference and Noise Ratio (CINR) value for an initial Modulationand Coding Scheme (MCS) level in the open loop power control mode and arequired CINR value for a last MCS level in the closed loop powercontrol mode.